A Jewish man’s seminal fluid is impure and anyone who touches it is impure. To be purified, the one who touches it must immerse in a ritual bath.
Even when a Jewish man’s seminal fluid is ejected from the sexual organs of a Jewish woman, after they have had sexual relations, it can impurify one who touches it for up to three days from emission, and the woman is considered impure as well. Chazal set a limit of three days because up to three days the man’s semen can still inseminate, and therefore one who touches it is impure, but afterwards it loses this power.
One of the Talmudic sages asks about this: what is the rule if a Jewish man had sexual relations with a non-Jewish woman and she emits the seminal fluid after three days? Is it only seminal fluid in the womb of a Jewish woman which heats up, putrefies, and loses the power to inseminate? Because of the Jewish woman’s concern for the fulfillment of the commandments her body might heat up more than that of a non-Jewish woman’s. Or, perhaps, because non-Jews eat insects and reptiles their bodies, too, heat up, and the semen loses its power to inseminate as it does in the body of a Jewish woman.
The same sage goes on to ask: what is the rule for the seminal fluid of a Jewish man who had sexual relations with an animal? How long does the semen which is ejected from the animal’s womb impurify a person who touches it — three days as from the womb of a Jewish woman, or even after the three days, since a woman has a fore-uterus which causes the semen to lose its power to inseminate, but an animal has none and so does not cause the semen to go bad?
The Talmud leaves these doubts with no decision.
(Babylonian Talmud, Tractate Shabbat 86b)